Should you meditate with a timer?

You should meditate as much as possible. I session is ideally 1 hour.

Having said this, the best way to do this is by strong resolution.

You make a strong resolution as I will to this for 15 minutes. As you go on you will notice that the time you take is as much as you make the resolution. But be realistic when you start doing this.

when do you feel you should cut it off?

when it is not useful. For example, when you decide to sit for an unplanned period of time, and you find the noise of the alarm becomes a distraction.

You can train the mind to know when time's up.

You make the resolution "I will sit 30 minutes" and then you sit and when you feel 30 minutes are up you get up, go in another room and check how much you sat. Let's say you sat 24 minutes today. Almost the goal and the mind registers this.

The next day you make the same resolution "I will sit 30 minutes" and then when you feel the time is up you get up, go in another room and check how much you sat. Again, the mind register this and makes adjustments.

After some days of training the mind will know by itself when it is the time to get up.

Use a timer. It's one less thing your mind has to concern itself with. I find it utterly distracting to look at my watch to see if a session has elapsed. Eventually, when you start sitting for an hour or more, that break in concentration becomes significantly distracting and disruptive. Usually deep samadhi is the only thing keeping leg pain (for example) at bay. It also fragments the concentration you've worked so hard to establish!

You can just use an egg timer (preferably the non-ticking type). There are also a bunch of smartphone apps. I prefer the former as there are no guarantees that someone isn't going to text you twenty minutes in!

I have been using a meditation timer with great success (There are a number of meditation APPs you can leverage). I actually started with a 5 minute timer (every morning) and since have added a minute each week as I build up to 30 minutes. To me, it feels like slowly incrementing the length of meditation has kept me from internally clock watching.

I'll echo other peoples comments by saying that trying to peek at a watch or think about the time would be pretty distracting and take away from the actual meditating.

Use a timer for the following reasons.

• Worrying about whether your session is done is a distraction. A timer will free you from this.

• A resolution to sit for X minutes may not work.

• Even if the resolution does work, you could end up mistaking that for progress. Don't laugh -- people mistake side effects of practice for the goals of practice. They'll ask if meditation can improve their grades, lead to particular hallucinations, etc...
• There's no harm to using a timer. After all, group meditations also use a timer, it just happens to be a human who hits a chime. There's nothing magical about people that makes chimes hit by them any less disruptive than those generated by an electronic device.

Meditate with timer is very rare in Theravada Buddhism. During meditation period, you might only need to focus on breath in and breath out. This time you do not need to hear, you do not feel pain, you do not need to be happy, you do not need to think about something, you do not need to think about the timer. This time you can feel everything is neutral. You only know the breath in and breath out. Meditation with a timer does not calm your mind. You worry when the time is finished, you worry you will miss the alert sound. Something you worry or something in your mind is not a good practice of meditation. So I personally suggest, do meditation when you have enough time.